This invention relates to an apparatus for testing materials under combined static and dynamic loads.
Materials used in rotating components of, for example, jet engines are subjected to unusually severe environmental conditions during their operational lifetimes. Components such as disks and blades are subjected to high temperature excursions, high levels of "steady" stress from centrifugal loads (low cycle stress), as well as high levels of high frequency "dynamic" stress from resonant vibration response (high cycle stress). The thermal-mechanical cycling of the component material can lead to initiation and propagation of cracks at voids, defects or scratches. The crack initiation and propagation characteristics of materials used in jet engine components vary widely, according to whether the material is more or less ductile, and depend on temperature, low cycle stress, high cycle stress, frequency, dwell time, local atmospheric conditions and many other parameters. Test methods to obtain data vary widely, but are generally limited in one way or another. In particular, few test systems can independently vary low cycle and high cycle loads over a wide frequency range, even in accordance with a simplified cycle, such as (1) apply low cycle load, (2) apply high cycle load, (3) discontinue high cycle load, and (4) discontinue low cycle load. Most test systems are limited to loads of a few hundred hertz. For example, many servo-hydraulic systems can combine low cycle loads with high cycle loads at frequencies up to 10 or 20 Hz. A few systems can operate at frequencies up to about 500 Hz, or marginally higher. However, none of these systems is simple or low in price. There exists a need for a simple, low-cost fatigue test system for investigating crack initiation and propagation behavior in test specimens under combined low cycle and high cycle loading, which can operate at very high frequencies.
Accordingly, it is an object of the present invention to provide a novel fatigue test system which satisfies the above-stated need.
Other objects, aspects and advantages of the invention will be apparent to those skilled in the art.